Phase behavior of heptaethylene glycol dodecyl ether and its aqueous mixture revealed by DSC and FT-IR spectroscopy

The phase behavior of heptaethylene glycol dodecyl ether (C12E7) and its aq ueous mixture was investigated by means of differential scanning calorimetr y (DSC) and Fourier transform infrared (FT-TR) spectroscopy in the temperat ure range from -20 to 70 degreesC. Phase boundaries among various phases in cluding mesomorphic phases were determined on the basis of DSC thermograms, from which the binary phase diagram of this mixture system was constructed . According to the phase diagram, the mixture viith the composition of 31 w t % H2O exhibits the following phase sequence with increasing temperature: solid --> H-1 (normal hexagonal) --> V-1 (normal bicontinuous-type cubic) - -> L-alpha, (lamellar)--> liquid. FT-IR measurements for the mixture with t his composition revealed the following features concerning the conformation al structure of the C12E7 molecule and the interaction between the surfacta nt and D2O in each phase. In the solid phase, the polyoxyethylene (POE) cha in of the surfactant molecule has a rather extended structure with a trans- rich conformation, in contrast to the case of pure C12E7, in which a helica l structure is dominant. When the solid phase transforms to the H-1 phase, the hydrogen-bond interaction between C12E2 molecules and between C12E7 and water are both weakened, and the fraction of gauche-conformer in the POE c hain increases, whereas the alkyl chain adjacent to the POE chain remains, taking a rather trans-zigzag conformation. The enhanced fraction of gauche- conformer in the POE chain suggests that the POE chain has a helical struct ure in this phase. The increase in temperature within the H-1 region causes a conformational change in both the POE chain and the alkyl chain of the s urfactant to a more disordered structure. Among the phases of V-1, L-alpha, and liquid, no significant difference is seen in the conformational struct ure and interaction with water molecules of the surfactant.